I have a question, how could you manage that only companies can place an ad but no normal users? I need your help there.

I would have thought of something like that, but it doesn't work, the page then has an error 500.

<?php if(osc_user_is_company()) { ?>
Maybe, you have an idea.

Best regards
Roberto

Get logged user record and use b_company key

And how do I then build one in the header?

So I did it like this in the header, I hope it should work.

<?php $seller = User::newInstance()->findByPrimaryKey(osc_logged_user_id());
if ($seller['b_company'] == '1') if( osc_users_enabled() || ( !osc_users_enabled() && !osc_reg_user_post() )) { ?>
          <a class="publish isTablet isDesktop" href="<?php echo osc_item_post_url_in_category() ; ?>"><?php _e("Publish Ad", 'sigma');?></a>
        <?php } ?>
installed the code.

$seller = User::newInstance()->findByPrimaryKey(osc_logged_user_id());
if ($seller['b_company'] == '1')
It might also be of interest to others.

Yes.
In latest version osclass there is also osc_logged_user() function that also returns row.